1. Field of the Invention
This invention relates to a signal transmission device, and more particularly to a signal transmission device for transmitting signal between a transmitting unit and a receiving unit which are rotated relatively to each other in a non-contacting manner.
2. Description of the Prior Art
Hitherto, between a transmitting unit and a receiving unit which are separately constructed and rotated in a non-contacting manner, a signal is transmitted by wireless transmission system utilizing transmitting and receiving antennas. However, in a wireless transmission system, intermixture of external noise is unavoidable, and therefore such a system is unreliable in accurate signal transmission. In this view, inventors of the present invention have proposed a signal transmission system for transmitting signal between a transmitting unit and a receiving unit which are rotated relatively to each other in a non-contacting manner. Such a signal transmission system is disclosed in Japanese Patent Applications Nos. 04-070640 and 04-291181. According to the signal transmission system mentioned above, signal is transmitted between transmitting unit and receiving unit in a non-contacting manner, and accordingly desired signal is reliably transmitted between units which are rotated relatively to each other.
However, in such a signal transmission system, there exist the following problems. Firstly, in the above system, signal is mainly transmitted by travelling wave component thereof which has wavelength corresponding to the frequency of signal to be transmitted. If magnitude level of the travelling wave component received by the receiving unit is not large enough, signal cannot be appropriately transmitted.
Secondly, there is such a problem that the gain of transmitted signal is not flat within the transmission frequency band thereof. Namely, the gains of the signal are different frequency to frequency, and peak points and bottom points of the transmission gain are repeatedly appears along a frequency axis. Accordingly, when a plurality of signals are modulated into different carriers within the transmission frequency band and then transmitted, a number of signals to be transmitted are limited by the transmission characteristics because the carrier frequencies have to be selected from the peak point frequencies where the transmission gain is sufficient. In addition, at frequencies slightly deviated from the peak point frequencies, the transmission gain is suddenly decreased and therefore transmission accuracy is deteriorated even if the peak point frequency is used as a carrier.
Thirdly, in such a signal transmission system, it is desired to perform bi-directional transmission. For example, in an inspection system in which objects are photographed by inspection cameras and quality of the objects are judged based on the image signals, inspection image signal is required to be transmitted from a detection unit to a signal processing unit while synchronizing signal for controlling the inspection cameras is required to be transmitted from the signal processing unit to the detection unit. In such a case, two signal transmission systems are prepared for the image signal transmission and the synchronizing signal transmission. However, in some kind of systems, it is difficult to provide a plurality of signal transmission systems. In addition, if bi-directional transmission is realized by a unique transmission device, the construction of the whole system and control of the system are simplified.